Electrical wave filter



j 1,636,737 July 25 1927' E. DlETzE ELECTRIGAL'WAVE FILTER Filed Sept. 8, 1921 2 Sheets-Sheetl Munn L gg [e 2 if', fly;

IMJ 205/3' 2 by@ o fwk--fwi-mw-a Y 1 l a j 5f l Kfz/6 C T yT INVENTOR ATTORNEY July 26, 1927. 1,636,737

' E. DIETZE ELECTRICAL wAvE FILTER Filed Sept'. a. 1921. 2 sheets-Sheet 2 E S f Q t,\m

equezwy llf'equezwg Patented July 26, 1927.

UNITED STATES PATENTl oFFl-CE.

EGINHARD DIETZE, Ol' BROOKLYN, NEW YORK, ASSIGNOB TO AMERICAN TELEPHONE AND TELEGRAPH OOIPANY, A CORPORATION OF NEW YORK.

ELEOTBICAL WAVE FILTER.

application mea september s, naar.l serial no. 499,191.

The principal object of my invent-ion is to proyide a new and improved wave filter having certain desirable o erating characteristics. Another object o my invention is to provide a modified single bandv filter that shall have a sharp cut-off between the freely transmitted frequency range and the attenuating ranges on both sides. Other objects of my invention will become apparent on consideration of the following specification in which I have specifically disclosed a few embodiments with the understanding that the scope of the invention is defined in the appended claims. I now proceed to a specific description of the examples of my invention which I have chosen to illustrate in the drawings.

Figure 1 is a diagram showing a wave filter of general type, Fig. 2 is av diagram corresponding to Fig. 1 and having certain modifications to facilitate explanation; Fig. 3 is a diagram of a confiuent band type of wave filter; Fig. 4 is a diagram of another type of band pass wave filter; Fig. 5 is a diagram of a composite filter built from elements shown in Figs. 3 and 4; Fig. 6 corresponds to Fig. 5 but has certain impedance elements consolidated; and Figs. 7, 8, 9, 10, 11 and 12 are attenuation-frequency diagrams corresponding to Figs. 3 to 6 and modifications thereof.

In presenting the theory of wave filters, itis customary first to consider a network like that shown in Fig. 1 with series impedances e, 'and shunt impedances z, repeated indefinitely, or in other words extending to infinity It is assumed that these impedances z, and z2 are pure reactances, for the reason that the dissipation' losses may be neglected with approximate accuracy for many purposes. In practice a reasonable number of the elements a, and z2 are used, and terminal networks are provided that make Vthe impedance of the finite structure approximately the same as that of the infinite flten Since an impedance element such as z, is equivalent to two impedances each 1/2 s, yin series, it will be evident that Fig. 2 can be substituted for Fig. 1. The pointsV 1 and 2 or 3 and 4 which divide the series impedance elements are called mid-series points and the section between them is a midseries-section. Since the :structure extends to infinity, the impedance across the points 1 and 2 looking 'to the right will be the same as i across the'points 3 and 4 looking to the right. This is the `mid-series iterative impedance and will be designated Z. Accordmgly we have the impedance equation Zaz +z,+1/2z,+z

whose solution is The critical or cut-off frequencles of a filter are known to be given Yby the equations Z2 0 (2) and given by i Y f1 a Vfz (5) where fo and f3 are the critical frequencies at the non-,adjacent ends of the freely transmitted frequency ranges.I Equationv 4 is derived from equation 2. E uation 3 eolved` for' f has two solutions whic give f, and fr These two'solutions with equation 4 give usI three design equations toward the determination of L1, L2, C1 and C, from the preassigned critical frequencies f., and f3. One more condition is re uired fully to define L1, L2, C, and C2. t the impedance at the mid-fre uency 1 be Zm. Fromequation 1 it can be s own t at Y.

which gives the requisite fourth condition to determine L1, L C, and-C2.

ranges i vlos It can also be shownyk from equation 'l that In any filterof the general type here considered, l' y where a-l-zfis the propagation-constant and a is the, attenuation constant.. ASeparating this equation into its real 'and imaginary parts, a can be expressed as a function of frequenc of which Zl and Z2 are func: tions. ig. 7 isa graph of the relation of a and f for the filter of Fig. 3; in other` words, Fig. 7 is the attenuation-frequency characteristic for the' confluentLband 'filter of Fig. 3. Thev other figures 8 to 12 similarly are attenuation-frequency characteristics of filters and are based on the equation y Now consider Vthe alter of Fig. 4.; yThis has a single band of transmitted frequencies whose two cut-ofi' frequencies fo and f3 are given respectlvely by the solutions of-equations 2.and 3. Athird design condition for L1', L2', 0, yand C2 is `given by an equation corresponding to equation 6, namely Zm Cz' 2C1I21ffo'fa' (8) From physicaly consideration it is seen that at the resonance frequency f2 for each shunt vcombmation L2 C2 there must, be infinite l attenuation. This glves av fourth condition,

namely 'and it is seen that we are freeto" put this] frequency f2', of infiniteattenuation'f ,where we please. Fig.f`8shows it close fto", .thef` upper cut-off frequency, and Fig."9f's ows it 1 ,close to f6', the lowerv cut-off frequency.

` lComparison of Fi s. 8 and 9 are for thefilter of Fig. 4.v

rom equations 1 and 8 it .can be shown 'that' the impedance for the filter of. Fig. 4

1s given by- 10 vfl caf-foe i if for 'the two ters of,-. Figs. 3 and. 4, we make the cut-off frequencies-the same, that 1s 1f we m'ake-foz/Jl and fszfs and if we make the lmpedances the same at mid-frequency4 y l,

fwvfofz, i

that is if we make Zm=Zm; vthen the im` pedances are the same over the entire fre- Arespect to the mid-frequency of the transmisison band. The combination 'ves va band filter with good selectivity ony th sides of the free range, but the attenuation y' assumesrelatively low values for frequenetqluations-i' and 10 shows that y l. I

' ree transmission '-offilt'ers of yFlgs. 3 and 4.

these `characterlstics may be obtainedby the v use-'ofdiierent numbers of sections of each .l

naamw by'adding the ordinates yof Figs. 7, 8 and 9 or any two of these. Fig. 10 illustrates of'Flgs. 3 and 7 is combined with the filter of Figs. 4 and 8. It will be 'noted that the combination sharply discriminates against frequencies at theright-hand edge` of the free transmission range, and also has high attenuation for frequencies considerably removed from the limiting frequencies. It will readily be seen that, by combining secthe case when the filter' tions of the filters of Figs. 7 and 8"anoth`er composite filter will be obtained, which is in everyrespect similar to the one whose characteristic is shown in Fig. 10 except that the sharp discrimination is on the lefthand side of the free transmission range.

Fig. 11 illustrates the case of a combination of two filters, each structurally like Fig. 4 but `with their parameters chosen so that their characteristics correspond respectively to Figs. 8 and 9. Itwill be seen from an inspection of this Fig. 11 that the atteny u'ation rises sharply on both sides of the freetransmission range. Choosing the fregillencies ofmaximum attenuation of the two tersan equal distance from the upper or respectively lower limitingy frequency of the free transmission band, a composite filter is obtained having an attenuation characteris tic' which is very nearly symmetrical vgth ee cies considerably 'removed from the edge of the free band. i

Fig.,12 shows the combination of a com posite Afilter ofthe e shown in Fig. 11

'ils

with'a` continent han `filter of Fig.` 7. This combination combines the advantages of sharp .discrimination 'on vboth sides of the free rangewith high attenuation for frequenciesf considerblyfi removed from the an a The Figs. 1o, 11 'ladliejiuusaathe characteristics of the different composite filters which may be obtained b.l

variation 1n type entering into the combinations. Thus,

combinationl r for; instance, if in the combination illusifk trated in Fig. 12, it is desired to obtain sharp selectivity at the edge of the band, this may be obtained by using a correspondingly larger number of sections of filters 6 according to Fig. 4 than of Fig. 3. Simifrequencies which it is desired completely4 to suppress. l Iclaim:

1. A band filter having sections of different type, the sections of one type being such that a complete filter of sections of that type will have a dierent attenuation frequency characteristic but the same cut-ofi' frequencies as a filter Whose sections are all of another type involved, whereby the composite lter combines the advantages of the different attenuation frequency characteristics.

2. A band filter having different kinds 0f sections, the sections of each kindbeing such that a complete filter of one kind of sections will have a different attenuation frequency characteristic from the others, but the same critical frequency and the same impedance frequency characteristic as the others.

3. A band filter having its cut-off sharp- `cned by the replacement of a section thereof by asection of different type whereby its attenuation characteristic 1s madeV steeper near its critical frequency, the said substituted section being such that a filter with all its sections like it will have the same critical frequencies. y Y

4. A band filter having sections of the same critical frequencies but different attenuation frequency characteristics, one section giving high attenuation over one frequency range and another section over another frequency range, whereby the composite filter gives high attenuation over both ranges.

5; A band filter having mid-series sections of two types of filters of the same critical frequencies and the same mid-frequency impedance but having different attenuation characteristics, whereby the composite filter. combines the advantages of both characteristics.

6. A band filter having four elements per section, one section being of confluent band type and another section having the same critical frequencies as the confiuent band section and the same mid-frequency impedance and alsohaving a frequency of maximum attenuation close to one of said critical frequencies.

In testimony whereof, I have signed my name to this specification this 1st day f September 1921.

EGINHARD DIETZE. 

